1. Field of the Invention
This invention relates to a new and improved chemical process for preparing a 1,1-dihalo-4-methyl-1,3-pentadiene, an intermediate in a known method for the production of certain pyrethroid insecticides.
2. Description of the Prior Art
Pyrethroids, naturally-occurring and synthetic derivatives of cyclopropanecarboxylic acid, have long been of interest as insecticides because they are active against a wide range of insect species, they display relatively low toxicity toward mammals, and they do not leave harmful residues. A notable recent technical advance in the pyrethroid art was the discovery of a dihalovinylcyclopropanecarboxylate, 3-phenoxybenzyl 2-(.beta.,.beta.-dichlorovinyl)-3,3-dimethylcyclopropanecarboxylate, having an outstanding combination of insecticidal properties [Elliott et al., Nature, 244, 456 (1973); ibid., 246 169 (1973); South African 73/3528]. Since Elliott's discovery, a great deal of interest has been generated worldwide in economical processes for the production of this type of pyrethroid.
Several years before Elliott's discovery, a method for the production of ethyl 2-(.beta.,.beta.-dichlorovinyl)-3,3-dimethylcyclopropanecarboxylate was reported [Farkas et al., Coll. Czech. Chem. Comm., 24, 2230 (1959)]. This compound leads to the Elliott pyrethroid by ester interchange [Nature, 244, 456 (1973)]. According to the Farkas method, chloral may be condensed with either isobutenyl magnesium bromide or with isobutylene, using a free radical catalyst with the latter, to produce a mixture of pentenols, which then may be reacted as follows: ##STR1##
By this method the overall conversion of isobutylene to 1,1-dichloro-4-methyl-1,3-pentadiene, the key intermediate for the diazotization step, is reportedly less than 40%. Furthermore, for every kilogram of 1,1-dichloro-4-methyl-1,3-pentadiene produced, more than a kilogram of zinc dust is consumed. In a recent year, U.S. producers alone sold about 300 million kilograms of synthetic organic insecticides [Chemical and Engineering News, July 28, 1975, p. 19]. If the Elliott pyrethroid becomes a major agricultural commodity, commercial production of 1,1-dichloro-4-methyl-1,3-pentadiene by the Farkas method would seriously tax the world supply of zinc. Thus, other more practical and economical processes capable of yielding 1,1-dichloro-4-methyl-1,3-pentadiene from readily available starting materials have been sought.
As Farkas recognized, dienes of the type represented by 1,1-dichloro-4-methyl-1,3-pentadiene generally are prepared by the alkaline or pyrolytic dehydrohalogenation of 1,1,1,3-tetrachloroalkanes, which, in turn, are produced by the condensation of an alkene with carbon tetrachloride in the presence of a free radical catalyst [M. S. Kharasch et al., J. Amer. Chem. Soc., 69, 1100 (1947); ibid., 69, 1105 (1947)].
Following the general methods, Farkas attempted to make 1,1-dichloro-4-methyl-1,3-pentadiene by the dehydrohalogenation of 1,1,1,3-tetrachloro-4-methylpentane, but his efforts along this line were completely unsuccessful. Although the condensation between carbon tetrachloride and 3-methyl-1-butene proceeded in 38% yield, the treatment of 1,1,1,3-tetrachloro-4-methylpentane with base failed to produce a detectable amount of the desired 1,1,-dichloro-4-methyl-1,3-pentadiene. Furthermore, pyrolytic dehydrohalogenation of 1,1,1,3-tetrachloro-4-methylpentane "led only to dark colored tars" [Coll. Czech. Chem. Comm., 24, 2230 (1959)].
Thus, Farkas was compelled to adopt the costly, commercially unattractive method diagrammed above for the synthesis of the 1,1-dichloro-4-methyl-1,3-pentadiene required to produce the cyclopropanecarboxylate.